The present invention relates to a control apparatus for an electric power steering system.
An electric power steering system for vehicle detects a steering torque generated at a steering shaft by operating a steering wheel and a vehicle speed, calculates a steering assist instruction value on the basis of the detected signal and assists a steering force of the steering wheel by driving a motor in accordance with the calculated steering assist instruction value where an electronic control apparatus including a microcomputer is used for calculating the steering assist instruction value and controlling the motor on the basis of the steering assist instruction value.
In this case, in respect of the steering torque, there are a component in correspondence with road load generated by the steering and a component in correspondence with a frictional force of a steering mechanism. Therefore, there has been proposed a control apparatus for calculating the steering force assist command value by adding a control value in correspondence with the road load determined on the basis of the detected steering torque and a control value in correspondence with the frictional force of the steering mechanism.
According to the constitution, the control value for the road load in correspondence with the steering torque and the control value for the frictional force in correspondence with the steering torque are previously determined and stored in a memory and desired data is read from the memory in correspondence with the detected steering torque whereby the steering force assist command value is calculated (refer to Japanese Examined Patent Publication No. JP 5-10271).
According to the above-described control apparatus for calculating the steering force assist command value, the control value for the road load and the control value for the frictional force are predetermined in accordance with the steering torque and therefore, when the steering torque is determined, the steering force assist command value changes only in correspondence with the car speed.
Accordingly, when the steering force assist command values in correspondence with the steering torques are previously set and stored in a memory with respect to a plurality of the car speeds, the steering force assist command values can be calculated immediately from the detected steering torques and the detected car speeds.
FIG. 10 is for explaining a conventionally known method of setting the steering force assist command value in correspondence with the steering torque with respect to car speeds V1, V2 and V3 where the steering force assist command values I in correspondence with the detected values of the steering torque T1, T2, T3 and T4 are set in accordance with respective car speeds. As shown by FIG. 10, for example, when the car speed is V1, the steering force assist command value I1 is set for the steering torque T1, I2 for T2, I3 for T3 and I4 for T4, and when the constant steering force assist command values I are stepwisely set such that the steering force assist command value I1 is set in the range of the steering torque of T1 through T2, the steering force assist command value I2 in the range of the steering torque of T2 through T3, and the steering force assist command value I3 in the range of the steering torque of T3 through T4, the capacity of a memory can be reduced. However, when the steering force assist command values I are set stepwisely, the steering force assist command values I are not changed continuously in compliance with changes in the car speed and the steering torque and accordingly, the steering assist force is not changed smoothly whereby the steering feeling is deteriorated.
As a measure therefor, the steering force assist command value may be set as finely as possible in correspondence with the steering torque and the car speed, however, a necessary capacity of a memory is significantly increased by this method, resulting in an increase in cost.
Accordingly, the applicant has proposed a method where the steering force assist command values in correspondence with the steering torques are stored in a memory in respect of representative car speeds, when the detected car speed is determined to be at an intermediary of the representative car speeds stored in a memory, the steering force assist command values in correspondence with the steering torques with respect to the representative car speeds faster and slower than the detected car speed, are read from a memory means and the steering force assist command value corresponding to the detected car speed and the detected steering torque is calculated on the basis of differences thereof with respect to the detected car speed and a car speed correction coefficient (Japanese Laid-Open Patent Publication No. Heisei 8-15095).
However, even with the method where the steering force assist command values in correspondence with the steering torques are stored in a memory with respect to the representative car speeds, when the steering force assist command values in correspondence with the steering torques are finely set, a necessary capacity of the memory is still increased, resulting in an increase in cost. Moreover, when the steering force assist command values are changed, stored data of the memory must be changed which requires time and labor.
As a further problematic point, when the steering force assist command values in correspondence with the steering torques are calculated, in calculating by a limited word length of, for example, 8 bits, lower digits (8 bits) of a result of calculation (constituting of 16 bits) are omitted whereby a quantization error on the basis of a digital calculation is caused. Such a quantization error is not preferable since discontinuous steering feeling is given to a driver when a slow steering operation is conducted.
That is, according to the above-described electronic control circuit for an electric power steering system, a low resolution circuit dealing with 8 bits signals is generally used, even if a result of calculation by a microcomputer is of 16 bit signals, in the case of a low resolution circuit where a PWM signal processing circuit or an A/D (analog/digital) conversion circuit deals with 8 bits signals, whether 0 is to be outputted or 1 is to be outputted cannot be determined as an output value when signals smaller than the resolution are dealt with and accordingly, the output signal is vibrated with a resolution of substantially 1 bit. This is referred to as limit cycle vibration.
Such a vibration includes a vibration component in the steering force assist command value and is felt as an unpleasant vibration in slow steering operation.